Electrical control circuit



Nov. 28, 1944. F. J. soMERs ELECTRICAL CONTROL CIRCUIT Filed. Dec. 27, 1941 04 f M 9 x 2 4% M 7 MM 5 mw ,4 BR/GHTA/FSS LONE/VS Pm so. F7. 5- INPUT V008 r w w m W O R T. wkwm m F Patented Nov. 28, 1944 ELECTRICAL CONTROL CIRCUIT Frank J. Somers, Rockville Centre, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 27, 1941, Serial No. 424,645

9 Claims.

This invention relates to electrical control circuits and more particularly to the method and apparatus for controlling peak-to-peak ratios in amplifier systems containing non-linear am- In certain amplifying systems, such as, for example, those used in television, it is desirable to deliberately introduce non-linear amplification in order to modify the signals representing tone gradations in the picture to be reproduced. Thus, for example, if the picture contains a shadow area in which it is desirable to reproduce all the detail in the area, then the signals representative of the shadow area are expanded or amplifled in a greater proportion than the signals in the white" portion of the picture. Alternatively, it may be desirable to suppress the shadow" detail entirely and this can be done by reducing the amplification of the signals representative of "black and increasing the amplification to a greater degree of the signals representative of white. Unfortunately, the introduction of such a control of amplification, commonly called in television, gamma control, changes the ratio of the peak-to-peak input signal voltage to the peak-to-peak" output signal voltage. The change in ratio results in improper reproduction unless corrective means are introduced to hold the ratio constant regardless of the value of gamma control. While this signal amplitude correction can be accomplished manually, such control is unsatisfactory since the operator, who may be at a remote control point, has no information except his judgement of a reproduced picture as to how much correction to introduce. In addition, it it diflicult to follow manually any rapid changes in signal level. My invention overcomes the shortcomings of such manual control systems, since my invention provides automatic control to give a constant ratio of peak-to-peak input to output.

Furthermore, as is well known in television, certain types of television camera tubes have nonlinear response to .the intensity of illumination. Similarly the grid-voltage versus light-output characteristic of most television reproducer tubes have a non-linear shape. The overall result of combining such non-linear elements in a complete television system is to accentuate the contrast in the range corresponding to one group of light values to the detriment of reproduction oi. gradations in another group of light values. It thus becomes necessary to provide a type of amplitude distortion complementary to the video faithfully reproduced at the television receiver.

Such correction may be applied at one or more points in the video amplifying system.

In general, it is preferable to distribute the complementary correction so that part of it is applied at the transmitter and part of it is applied at the receiver. Thus,-that part of the video signal corresponding to the darker parts of the picture can be made to give a higher average percentage modulation if the correction is split between the receiver and the transmitter than if the correction were applied at only the transmitter or the receiver. There results from this division or correction an advantage in that the signal-to-noise ratio at the receiver can be improved. By providing automatic control to maintain the peak-to-peak signal ratio constant, the advantages of non-linear amplification can be enhanced. Accordingly, it is the main object of my invention to provide a new, useful and improved ampilifyln System.

Another obect of my invention is to provide a method and apparatus to improve amplifying systems.

Another object of my invention is to provide a method of amplification control such that changes in gamma can be effected. without dis-.

turbing the modulation conditions established at a transmitter.

A still further object of my invention is to provide a new method and apparatus for maintaining the peak-to-peak ratio of input to output voltage of a non-linear amplifying system constant.

Another object of my invention is to provide an improvement in television amplifying systems.

Other objects of my invention will become at once apparent to those skilled in the art upon reading the following detailed description when taken together with the drawing.

In the drawing, there are two figures of which Figure 1 showsgraphically the relation between brightness and output of a camera tube, as-well as the non-linear characteristic of an amplifier used in my invention; while Figure 2 shows schematically apparatus useful in practicing my new invention and which includes circuit arrangements for maintaining the peak-to-peak ratio constant.

Referring now to Figure 1, I have shown at l the characteristic tf a television camera tube for example, an Iconoscope, which relates the brightness to the output of the tube in volts. The abscissa is shown in units of brightness such as lumens per square foot, while the ordinate is in units'of volts output. The curve B shown at 3 gives the non-linear characteristic of a gamma amplifier and has for its abscissa units of volts input and for its ordinate, output in volts. It will be noted that the two curves have complementary curvature so that if the output of the camera tube shown at A is fed to a gamma amplifier having a characteristic B, the overall characteristic will be substantially linear. It will be observed, however, that a unit increment of input voltage at'B provides an incremental output whose value will vary with the mean value of the incremental voltage supplied at the input. For example, an input difference or increment ab produces an incremental output de, while an incremental input be equal to ab will produce an incremental output 6). It will at Once be apparent therefore that the ratio of i ab is considerably smaller than the ratio of It will thus be clear that the peak-to-Deak ratio changes with the value of the average brightness in the picture, which is represented by the direct current component in the video signal. Moreover, it will be appreciated that the characteristic curve of the gamma amplifier 1, may be altered and that such an alteration alone would also produce a change in the ratio of the two "peak-to-peak values. Since the peak to-peak value affects the percentage modulation at the transmitter, it is necessary to insure that the ratio of the peak-to-peak" input to the "peak-to-peak" output remains constant in order to insure that proper modulation of the transmitter is maintained. It is therefore one purpose of the present invention to provide means for altering the relative modulation ratios for various picture tonal .values while keeping the total transmission modulation constant.

Turning now to Figure 2, I have shown video amplifier receiving th input from the television camera, for example, or its pro-amplifier, and feeding its output to the gamma amplifier i. The gamma amplifier is an amplifier having a characteristic 3 shown in Figure 1. The output of the gamma amplifier is fed to an automatic volume control amplifier 9 which in turn has its output connected to a secondvideo amplifier il. The output of this video amplifier is then fed to a transmission line to be further amplified and transmitted by the video radio transmitter. A portion of the output energ of the video amplifier 5 is fed through the condenser |3 to the amplifier tube l5 provided with suitable biasing and decoupling networks comprising the resistors 6|, 63, 61, 69 and the condenser 65. The amplifier tube l5 has a gain of approximately unity, and operates primarily as a phase inverter tube, out-of-phase potentials being available from the anode and cathode of the tube. Push-pull signals available from the cathode and anode of tube l5 are applied to a bridge type rectifier 23 through the capacitors l9 and 2|, the capacitors l9 and 2| serving to avoid any D. C. connections between the tube l5 andthe other parts of the control circuit.

The polarity of the rectified voltage is as indicated in Figure 2. The rectified voltage appears across the networkincluding the condenser 39 and the resistor 4| which network is connected between the diode 31 and the network comprising the parallelly connected condenser 43 and resistor 45. There is thus provided across the terminals of the network 39, 4| a D. C. potential which is proportional to the peak-to-peak" video voltage output of the amplifier 5. A portion of the output energy of the video amplifier H is similarly fed through condenser 25 to a thermionic amplifier and phase inverter tube 21 which is also provided with suitable biasing and decoupling network including the resistors 1|, I3, l1, l9 and the condenser 15. The energy appearing at the anode and cathode of tube 21 is fed through the condensers 3| and 33 to the bridge rectifier 35 to produc a D. C. potential of the polarity as indicated in the Figure 2. This D. C. potential which is proportional to the peakto-peak voltage output of the amplifier II is supplied to the anode of the tube 31 and to the junction point of the parallelly connected condenser 43 and resistor 45 connected to the biasing voltage. From the common junction point of the networks 394| and 43-45 a connection is brought to the amplifier tube 41 whose plate is supplied by voltage through the resistor 49 and from the plate of which a low pass filter comprising resistors 5|, 53 and condensers 55 and 51 is connected. The output of the filter feeds the automatic volume control amplifier '9. The

amplifiers 9 and II are initially. adjusted so as to provide excess gain over that required to supply a signal to the transmission line suflicient to modulate the transmitter one hundred percent. In operation then, the voltage across the rectifier 35 will exceed the voltage across the rectifier 23 and overcome the bias produced by the rectifier 23 across the network 394|. As a result, current will fiow from the rectifier 35 through the loop including the impedances 43 and 45 producing, at the junction point of the two impedance networks, a positive voltage which is supplied to the grid of the tube 41. The tube 4! thus draws more current, producing an I. R.

drop in the resistor 43, so as to decrease the voltage supplied to the automatic volume control amplifier 9. As is well known, under these conditions, the gain of the amplifier 9 will be reduced and this reduction in the gain of the amplifier 3 will take place up to the time that the voltages produced b the rectifiers 23 and 35 balance each other so that the potential on the grid of the tube 41 maintains the gain of the amplifier 9 to keep the two rectifiers in balanced condition. It will, of course, be appreciated that the time constants of the impedance networks 39-4| and 43-45 and the low pass filter 5|53, 555'l are chosen, so as to insure that the change of the gain of the amplifier 9 takes place over one or more frames of the pictures to avoid abrupt changes in brightness in the received picture. Also, in the interest of good design, it is desirable while maintaining the proper R. C. time constants to make resistor 45 about one hundred times as large as resistor 4| such that when rectifier 31 draws current, the major portion of the potential drop will be produced across the terminals of the R. C. network 43, 45. Under these conditions, the current through the tube 31 will not appreciably alter the potential acros the impedances 39 and 4| so that this potential continues to represent the peak-to-peak input voltage to the ampliher 5. As an example, practical values for a time constant of one-tenth of a second would be Resistor 4l=20,000 ohms Condenser 89:5 mi'd.

Resistor 45=2 megohms Condenser 43=0.05 mfd.

to the gamma amplifier 1 is equal to the peakto-peak voltage at the output of the video amplifier H.

Again, it will 'be noted that the units 5, l, 9 and II are conventional ones ordinarily included in a television chain, and consequently, the only additional apparatus required for affecting the automatic control of the ratio of peak-to-peak. voltages of the tubes i5, 21, 41 and 31 and their associated components. Accordingly, these elements of the control circuit may be built into one unit which can be added to any existing television amplifier chains to improve operation. It will be appreciated, of course, that while the tubes I5, 21 and 41 have been shown as triodes, this is merely by way of example and pentodes or any other thermionic amplifier tubes may be used. Likewise, while the tube 31 is shown as.

a diode, it will be clear that a triode type rectifier may be used and this is also true of the rectifier bridges 23 and 35. Again it will be appreciated that while amplifiers and II have been shown in Figure 2., these amplifiers may be omitted in those 'cases where the signal level is suflicient to give ample rectified voltages for comparison with each other.

Various alterations and modifications of the present invention may become apparent to those skilled in the art and it is desirable that any and all such modifications and alterations be consid-' ered within the purview of the present invention except as limited by thehereinafter appended claims.

What I claim is:

l. The method of maintaining a constant ratio between the peak-to-peak value of the ener y at the input and output of a non-linear amplifier system, which includes the steps of supplying wave energy to be amplified, amplifying the supplied energy non-linearly with respect to the amplitude of the supplied energy, amplifying the non-linearly amplified energy, deriving energy proportional to the peak-to-peak value of the supplied energy, deriving energy proportional to the peak-to-peak value of the non-linearly amplified energy. comparing the two derived energies and controlling the amplification of the non-linearly amplified energy in accordance with the difference between the compared energies.

2. The method of maintaining a constant ratio between the peak-to-peak value of the energy at the input and output of a non-linear amplifier system, which includes the steps of supplying wave energy to be amplified, amplifying the supplied energy non-linearly with respect to the amplitude of the supplied energy, amplifying the non-linearly amplified energy, deriving a direct current potential proportional to the peak-topeak value of the supplied energy, deriving a direct current potential proportional to the "peakto-peak value of the non-linearly amplified energy, comparing the two derived potentials and controlling the amplification of the non-linearly amplified energy in accordance with the difference between the compared potentials.

' to-peak 3. The method of maintaining the peak-to-- peak ratio of the input and output energy of a non-linear amplifier system constant, which includes the steps of supplying wave energy to be amplified, amplifying the supplied energy nonlinearly with respect to the amplitude of the supplied energy, amplifying the non-linearly amplified ener y, full wave rectifying a portion of the supplied energy to derive a potential proportional to the peak-to-peak" value of the supplied energy, full wave rectifying a portion of the non-linearly amplified energy to derive a potential proportional to the peak-to-peak value of the amplified non-linear energy, comparing the two derived potentials and controlling the amplification of the non-linearly amplified energy in accordance with the'difference between the compared potentials.

4'. A constant input. to output "peak-to-peak energy ratio amplifying system comprising means for supplying wave energy to be amplified, means for amplifying the supplied energy non-linearly with respect to the amplitude of the supplied energy, means for amplifying the non-linearly amplified energy, full wave rectifying means for deriving energy proportional to the peak-topeak value of the supplied energy, full wave rectifying means for deriving energy proportional to the peak-to-peak value of the non-linearly amplified energy, means for comparing the two derived energies and means for controlling the amplification of the non-linearly amplified energy in accordance with the difference between the compared energies.

5. A constant input to output peak-to-peak energy ratio amplifying system comprising means for supplying wave energy to be amplified, means for amplifying the supplied energy non-linearly with respect to the amplitude of the supplied energy, means for amplifying the non-linearly amplified energy, full wave rectifying means for deriving a direct current potential proportional to the peak-to-peak value of the supplied energy, full wave rectifying means for deriving a direct current potential proportional to the peak-to-peak value of the non-linearly amplified energy, means for comparing the two derived potentials and means for controlling the amplification of the non-linearly amplified energy in accordance with the difference between the compared potentials.

6. A constant input to output "peak-to-peak energy ratio amplifying system comprising means for supplying Wave energy to be amplified, means for amplifying the supplied energy non-linearly with respect to the amplitude of the supplied energy, means for amplifying the non-linearly amplified energy, means including a full Wave rectifier for rectifying a portion of the supplied energy to derive a potential proportional to the "peak-to-peak value of the supplied energy, means including a full wave rectifier for rectifying a portion of the non-linearly amplified energy to derive a potential proportional to the "peakvalue of the amplified non-linear energy, means for comparing the two derived potentials and means for controlling the amplification of the non-linearly amplified energy in accordance with the difference between the compared potentials.

7. A non-linear amplifying system comprising a source of signals to be amplified, a non-linear amplifier, a linear amplifier connected between said source of signals and said non-linear amplifier, means to amplify energy from said nonlinear amplifier, full wave rectifying means for deriving a potential proportional to the peakto-peak value of the energy from said source of signals, full wave rectifying means for deriving a potential proportional to the peak-to-peak value of the non-linearly amplified signals, means to compare the said derived otentials, and means to control the gain of said amplifying means in accordance with the diiference between the compared potentials.

8. A non-linear amplifying system comprising a source of signals to be amplified, a non-linear amplifier, a linear amplifier connected between said source of signals and said non-linear amplifier, means to amplify energy from said nonlinear amplifier, opposed unilateral conducting means for developing a potential proportional to the peak to-peak value of the energy from said source of signals, opposed unilateral conducting means for developing a potential proportional to the peak-to-peak" value of the non-linearly asaaais amplified signals, means to compare the two developed potentials, and means to control the gain of said amplifying means in accordance with the difference between the compared potentials.

9. A non-linear amplifying system comprising a source of signals to be amplified, a non-linear amplifier, a linear amplifier connected between said source of signals and said non-linear amplie fier, means to amplify energy from said nonlinear amplifier, a first full wave rectifying means for generating a potential proportional to the peak-to-peak" value of the energy from said source of signals, a second full wave rectifying means for generating a potential proportional to the peak-to-peak value of the non-linearly amplified signals, means to compare the two generated potentials, and means to control the gain of said amplifying means in accordance with the difference between the compared potentials.

RANK J. SOMERS. 

